Sampling apparatus for photoelectron spectrometry

ABSTRACT

Apparatus for vaporizing solid or liquid samples in a photoelectron spectrometer includes boundary walls of high thermal conductivity defining the sample oven cavity, which cavity includes both the zone in which the sample is vaporized and an adjacent zone in which the vaporized sample is irradiated by the monochromatic photon beam from, for example, a plasmadischarge photon source. The good thermal conductivity of the boundary walls causes the entire cavity to be maintained at substantially the same temperature, preferably just above the vaporizing temperature of the particular sample. The boundary walls may be heated substantially solely or partially either by a heater or by heat from the adjacent part of the plasma-discharge lamp, each of which is in good thermal contact with part of the boundary walls. The vaporized sample is thus formed as a slowly diffusing molecular cloud and condensation is avoided since the vapor never comes in contact with cooler zones, even upon entering the narrow photon passageway between the source and the sample cavity. Auxiliary cooling may be provided for maintaining parts of the photon source at a desired temperature, depending on the vaporizing temperature of the particular sample.

United States Patent Lempka [111 3,743,831 1 July 3,1973

[ SAMPLING APPARATUS FOR PHOTOELECTRON SPECTROMETRY [75] Inventor: Hans Joachim Lempka,

Beaconsfield, England [73] Assignee: Perkin-Elmer Limited, Beaconsfield,

England 22 Filed: Mar. 31, 1972' [211 App]. No.: 240,072

[52] US. Cl. ..250/425 [51] Int. Cl. "01] 37/26 [58] Fitld of Search 250/49.5 AB, 49.5 PE,

[56] References Cited OTHER PUBLICATlONS An Electron Energy Analyzer for Photoelectron Energy Distribution Studies On Solids, Feuerbacher et 211., Review of Sci. Inst., Vol. 42, No. 8, Aug. 71, pp. 1 172-1 173 Primary Examiner-James W. Lawrence Assistant Examiner-C E. Church Attorney-Edward R. Hyde, Jr.

57 ABSTRACT Apparatus for vaporizing solid or liquid samples in a photoelectron spectrometer includes boundary walls of high thermal conductivity defining the sample oven cavity, which cavity includes both the zone in which the sample is vaporized and an adjacent zone in which the vaporized sample is irradiated by the monochromatic photon beam from, for example, a plasma-discharge photon source. The good thermal conductivity of the boundary walls causes the entire cavity to be maintained at substantially the same temperature, preferably just above the vaporizing temperature of the particular sample. The boundary walls may be heated substantially solely or partially either by a heater or by heat from the adjacent part of the plasma-discharge lamp, each of which is in good thermal contact with part of the boundary walls. The vaporized sample is thus formed as a slowly diffusing molecular cloud and condensation 'is avoided since the vapor never comes in contact with cooler zones, even upon entering the narrow photon passageway between the source and the sample cavity. Auxiliary cooling may be provided for maintaining parts of the photon source at a desired temperature, depending on the vaporizing temperature of the particular sample.

21 Claims, 6 Drawing Figures Patented July 3, 1973 3 Shah-Shut l 5 Snob-Shut I Ti fiat Patented July 3, 1973 3 Shah-Shut 3 SAMPLING APPARATUS FOR PHOTOELECTRON SPECTROMETRY This invention relates to sampling apparatus for photoelectron spectrometry.

Photoelectron spectrometry is fast becoming one of the generally accepted physical methods of fundamental chemical research but so far its usefulness has been largely restricted to samples in the gas phase at room temperature. The extension of the method to the routine investigation of liquids and solids would represent a step forward of great practical significance.

In photoelectron spectrometry, highly energetic and monochromatic photon sources are used to bombard the molecules of a sample gas in a highly rarified atmosphere and eject from them a number of photoelectrons, the energy spectrum of which contains fundamental structural information that is retrieved by means of an energy analyzer, e.g., an electrostatic analyzer. High photon energy and monochromatism are required to insure sensitivity and resolution, respectively. The Helium I line is commonly chosen to meet these two requirements in current spectrometers, the practical source taking the form of a plasma discharge lamp in which helium gas at low sub-atmospheric pressure is excited by an electrical discharge passing therethrough. Because of the difficulty in producing windows transparent o the Helium I line, the photons are made to issue from a narrow output passage (i.e., an actual opening), which means that not only the lamp, but also the energy analyzer and the sample held, for example, in a sample probe must be operated under continuously maintained vacuum. The gas sample is exposed to the photon beam in a target chamber within the sample probe, the chamber being provided with an entry aperture for the photons and an exit aperture for the photoelectrons.

It is clear from the above outline that the vaporizing apparatus of any non-gaseous sample, of which only a very small quantity may be available, must in practice enable sample vaporization to take place in close proximity to the target chamber. Unfortunately, some vaporized samples tend to condense readily upon the walls of the chamber, with consequent contamination or total obstruction of either or both of said apertures. In more extreme cases, such as in handling samples vaporizing at comparatively high temperatures, condensation may also take place in and around the output passage of the plasma discharge lamp, with the result that the photoelectron spectrometer is either unexpectedly put out of action until the sample condensate has been cleared, or, worse, the next spectrum run with a different sample is spoiled by the presence of unsuspected residues of the old sample, too small to cause an obvious anomaly in the spectrometric results.

In theory, one could attempt to circumvent the condensation problem within the target chamber by causing the vaporized sample to condense on non-critical surfaces where it cannot affect the analytical results; in practice, such an inefficient use of the sample can not be tolerated without severely limiting the usefulness of the photoelectron spectrometer.

The primary object of the present invention is to provide a sample vaporization apparatus for photoelectron spectrometry generally and photoelectron spectrometers, in particular, in which sample condensation is minimized.

In combating sample condensation it has been found that the problem is greatly alleviated by providing a vaporizing apparatus in which a sample vaporizing zone is closely thermally coupled to a contiguous target zone by means of a surrounding thermally conducting heated boundary forming an almost complete enclosure. In practice, the two zones are made quite small in the interest of sample conservation, so that the heated boundary defines in effect a miniature oven cavity.

According to one aspect of the present invention there is provided an apparatus for vaporizing liquid or solid samples in photoelectron spectrometry, comprising: heat conducting boundary means for defining therein a miniature oven cavity providing a sample vaporizing zone contiguous to a target zone; a photon entry aperture and a photoelectron exit aperture in said boundary means, into and out of the target zone, respectively; and heating means in close thermal transfer relationship with the boundary means, whereby the completely surrounding heat emanating from the boundary means into the confined space of the miniature oven cavity assists the formation of a uniform molecular cloud diffusing with the random agitation of the individual molecules throughout the oven cavity and minimizes the risk of sample condensation.

In the sample vaporizing apparatus broadly outlined above, the heating means may or may not form an integral part of the probe. In the one alternative, the sample vaporizing apparatus may be furnished with, for example, a heating coil in good thermal coupling with the boundary means. In the other alternative, matters may be so arranged that when the vaporizing apparatus is positioned in a photoelectron spectrometer a part of the boundary means comes into good thermal contact with a heated member. In either case, molecules of samples vaporized in operation by the all-round heat transfer from the surrounding heated boundary means acquire random motion and drift into the target zone in the manner of a generally uniform cloud. It is believed that the diffuse molecular formation within a heated enclosure averages out differences in heat transfer from and to each molecule, so that, as long as the boundary means is sufficiently heated for the average temperature of the molecule to be maintained just above the vaporizing temperature of the sample, there is little tendency for any molecules to behave differently from the rest by condensing on the inner surface of the boundary means.

It must be admitted, however, that the vaporization of certain solid compounds, particularly some of those vaporizing at comparatively high temperatures, presents a particular problem in that, even if a high proportion of the sample molecules finallyescaping through the photon entry aperture and the photoelectron exit aperture of the probe can be led out to atmosphere through the vacuum pumping arrangement, some will inevitably escape and drift into the narrow output passage of the photon source. If the surface bounding the outer end of said output passage is at a temperature below that of the boundary means, the drifting molecules will tend to condense thereon and in the worst case completely obstruct said passage. The solution,

are kept on the move until they reach the hot plasma of the photon source where they are disintegrated and made harmless.

According to another aspect of the present invention there is provided an apparatus for vaporizing liquid or solid samples in a photoelectron spectrometer comprising: a heat conducting boundary means for defining therein a miniature oven cavity providing a sample vaporizing zone contiguous to a target zone; a photon entry aperture and a photoelectron exit aperture in said boundary means, into and out of the target zone, respectively', means for supporting a sample within the sample vaporizing zone; and a plasma-discharge photon source having a photon output member exposed at one end to the blast of the plasma discharge and making good thermal contact at the other with the boundary means, the photon output member being provided with a photon output passage which is aligned with the photon entry aperture and being so arranged in the photon source as to enable a substantial part of the heat generated by the plasma to be transmitted through the photon output member to the boundary means, whereby in operation plasma-generated heat is transmitted through the photon output member and the boundary means to the sample vaporizing zone and the temperature encountered by any sample molecules drifting from the sample vaporizingzone towards the plasma discharge is prevented from falling below the vaporizing temperature of the sample.

It has been found that a plasma-discharge lamp may be adapted for use in the dual role of spectrometric photon source and heatingmeans by adapting the lamp for operation at temperatures up to the highest plasma temperature that can be tolerated before the photon generating function is impaired and arranging for the member which normally provides the photon output passage, i.e., the photon output member, to receive adequate heat from the hottest plasma region and transfer it by conduction to a suitable part of the boundary means, which is contrary to the normal practice in the art, namely, of maintaining the photon output member very nearly at ambient temperature.

The heat output of the plasma increases with the electrical energy input to the source, and so does the Helium I output of the source, except that beyond an upper energization limit no significant incremental output of Helium I takes place. This is also true for the Helium II line, for example, but in this latter case the practical upper temperature limit'is much higher. Even in the case of Helium I generation, the upper limit is adequate in most cases for heating the output tube to the level required to avoid condensation. In extreme cases, an additional region of control of the output tube temperature by regulation of the lamp energization is afforded by the fact that the photon yield is not adversely affected if the energization limit of the source is substantially exceeded.

The plasma-generated heat conducted to the sample vaporizing zone can be made adequate for vaporizing a large number of samples, and where this is not practical or convenient additional heating means close to the sample vaporizing zone may be provided, together with means for regulating the heat output thereof.

If desired, the energization of either the source or the additional heating means may be servo-controlled for the purpose of attaining specified sample temperatures. Servo-controlling the additional heating means would have the advantage of greater simplicity, faster response and finer control. In fact, even where the photon source alone could supply the entire heat output required to vaporize the sample, it may be found desirable to limit the heat output of the source either through its electrical energization or controlled cooling in order that at least some heat is supplied by the additional heating means and advantage thus taken of the resulting finer temperature control.

It may be helpful to point out that the efficient utilization of a vaporized sample, especially if it vaporizes at very nearly ambient temperature, demands that the vapor pressure of the sample be raised no more than is enough to'keep a plug or cloud of randomly moving vaporized molecules in the target chamber for as long as possible before the molecules are exhausted from the miniature oven cavity by the continuous vacuum under which the oven cavity, the photon source and the analyzer of a photoelectron spectrometer must operate, for reasons mentioned earlier.

It should be observed that even when the photon source is assisted by the additional heating means, the heating effect of the plasma will still predominantly determine the temperature gradient between the hot plasma-discharge zone and the cooler sample vaporizing zone. Even if the additional heating means is arranged so that it tends to oppose the gradient, the result will be substantially removed from a complete reversal of the gradient, if the additional heat is suitably proportioned in relation to the main heat from the photon source.

If it is desired to use the apparatus of the present invention with a given sample the vaporizing temperature of which is slightly below ambient temperature, it may be found convenient to apply in the first instance localized cooling at the sample zone for maintaining the sample well below its vaporizing temperature and then gradually increase the plasma-generated heat, such as by increasing the current through the photon source; until the sample just begins to vaporize.

Although the specific structure of a vaporizing apparatus embodying the principles of the present invention may be adapted to satisfy particular requirements in photoelectron spectrometry generally, it will be found particularly useful in the application of the invention to an integrated photoelectron spectrometer instrument to incorporate the miniature oven cavity in a sample probe, preferably in a terminal part of the probe, or working tip, that can be easily brought by the user into and out of operational relationship with the input end of the energy analyzer of the spectrometer.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings, wherein:

FIG. 1 is a perspective view, partially cut away, of an exemplary sample vaporizing apparatus in accordance with the present invention, for use with a photoelectron spectrometer of the type comprising an electrostatic analyzer of electron energies;

FIG. 2 illustrates on an enlarged scale certain components of the FIG. 1 apparatus;

FIG. 3 is a modification of one of the FIG. 2 components;

FIG. 4 is a perspective view, with cut-aways, of a plasma-discharge source illustrating how it cooperates, in accordance with the present invention, with the FIG. 1 apparatus;

FIG. 5 is a modification ofa part included in the FIG. 1 apparatus suitable as an alternative of said part in the FIG. 4 embodiment, and

FIG. 6 is a simplified representation of the whole FIG. 4 apparatus, additionally schematically showing the various electrical supplies, controls and connections.

The vaporizing apparatus of FIG. 1 comprises a sample probe, generally referred to at 1, having a longitudinally extending cylindrical working tip 2 joined to a stem in the form of an outer stern tube 3 of preferably stainless steel. The working tip 2 is preferably obtained by boring out a solid copper rod from one end (the lower end in FIGS. 1 and 2) thereof to a predetermined depth so as to form a miniature oven cavity comprising sample vaporizing zone 2A contiguous to, and in fact merging into, a target zone 2B within a common longitudinally extending cylindrical wall 2AB, closed at one end by integral top wall portion 2D.

The copper rod is reduced in diameter at its other (upper in FIGS. 1 and 2) end and the resulting cylindrical portion 2C is forced fit into the outer stern tube 3, the joint being additionally brazed for vacuum tightness. The reduced diameter, end portion 2C is also bored out to a predetermined depth from its free end, the two bores in the working tip 2 being thus separated by the wall portion 2D (see especially the exploded and enlarged view of the working tip in FIG. 2).

In this upper bore fits a heater 4, which comprises a heater coil 4A bifilar wound on electrically insulating and heat-resisting former 4B supported in an inner tube 5 which is a slide fit in the outer stern tube 3. A temperature sensing coil of wire having a temperature coefficient of resistance is provided at 4C over the heater coil 4A, the leads for the two windings being taken through the adjacent end of the inner tube 5 to the opposite end thereof, where they are shown emerging from the inner tube 5. An alternative to the temperature sensing coil 4C is a thermocouple, which could be fitted to an axial bore in former 43.

Where the inner tube 5 carrying the heater coil 4A may have to be made interchangeable with a similar inner tube accommodating a cooling device, for a purpose that will be explained later with reference to FIG. 5, the heater coil may be surrounded by a cylindrical copper shield closed at the bottom and joined to the inner tube 5 at .the top, in order to minimize the risk of damaging the heater coil in the course of inserting the inner tube 5 into, or removing it from, the outer stern tube 3. In such case, the outer diameter of the cylindrical shield is naturally made a good slide fit into the bore of the reduced upper portion 2C of tip 2.

The open (lower) end of the working tip 2 is internally threaded to receive a combined screw plug and sample carrier machined from copper, generally indicat'ed at 6 (FIG. 1 and FIG. 2). Referring to the enlarged view in FIG. 2, a copper rod is machined from solid to define a screw plug part 6A extending into a hollow cylindrical part 68 having an elongated window 6C. The screw plug part 6A is provided with a fine axial passage 6D which from the center of the flat face 6E communicates with the bore in the cylindrical part 6B. The passage 6D constitutes a photon entry aperture intended for the purpose of allowing a beam of photons to penetrate axially into the region of the target zone within cylindrical part 68 while minimizing as much as possible the escape of vaporized sample. The longitudinally extending cylindrical wall 2AB, the top wall portion 2D and bottom wall portion represented by the screw plug part 6A of the combined screw plug and sample carrier 6 represent the boundary means of the entire sample cavity of the working tip 2.

The hollow cylindrical part 68 is intended to support at its castellated free end 6D a sample holder 7 shaped in the fashion of a short test tube, the nearhemispherical bottom of which locates in the bore of the cylindrical part 6B. According to a modification shown in FIG. 3, the free end of the cylindrical part 68 is provided with an enlargement 6F having longitudinal slits 6G. This arrangement forms a socket in which the sample holder 7 is slid against the radial pressure exerted thereon by the resiliently yielding portions between slits 6G in the enlargement 6F, the internal diameter of the enlargement 6F being slightly smaller than the outside diameter of the sample holder 7.

The dimensioning of the cooperating parts is such that when the combined screw plug and sample carrier 6 fitted with the sample holder 7 is screwed fully into the threaded end of the working tip 2, an annular gap remains between the rim of the sample holder 7 and the inner face of wall portion 2D through which the vaporized sample may diffuse from the holder 7 into the entire interior 2A, 2B of the cavity mentioned before.

The longitudinally extending wall 2AB of the working tip 2 is provided with a photoelectron exit aperture in the form of a fine longitudinal slit 2E through which photoelectrons ejected from the vaporized sample in the target zone 2B emerge into an electrostatic analyzer of electron energies (not shown). The arrangement is such that when the combined screw plug and sample carrier 6 is fully screwed into tip 2, as in FIG. 1, the window 6C is in register with the slit 2E. The knob 3A fastened to the outer stem tube 3 enables the operator to rotate the probe body about its longitudinal axis and find the optimum angular positioning of the slit 2E relative to the entry of the electrostatic analyzer (not shown).

In operation, the probe 1 is positioned in relation to a plasma discharge photon source in the photoelectron spectrometer so that a collimated beam of photons enters through 6D, and with a solid sample to be vaporized and analyzed located at the bottom of the holder 7, an electric current from an AC supply 20 (FIG. 6) adjusted by means of rheostat 19 is passed through the heater coil 4A while the temperature of the coil is monitored through the sensor 4C connected to a suitable instrument. The current is gradually increased from a suitably low value until whatever means are employed for observing the spectrometer output, e.g., a chart recorder, begins to respond. At that point, minor adjustments to the current may be made to regulate the rate at which the sample is vaporized and obtain a satisfactory signal-to-noise ratio in the electrical output of the energy analyzer. Obviously, too high a rate is undesirable: firstly, because only a very small quantity of sample may be available which must be made to last sufficiently long for a complete spectrum to be run, and secondly, because sample condensation troubles may be aggravated.

As soon as the solid sample begins to vaporize, molecules thereof will diffuse in a substantially uniform cloud from the sample vaporizing zone 2A into the target zone 2B, to which they gain access through the window 6C and the openings provided in the castellated end 6E (FIG. 2) or the equivalent slots 6H of the modified construction shown in FIG. 3. The formation of the uniform molecular cloud is to some extent assisted by the very low subatmospheric pressure set up in the miniature cavity (hereinafter sometimes referred to as the oven cavity).

It will be appreciated that no physical demarcation has been provided between the sample vaporizing zone 2A and the target zone 28. The target zone (i.e., where the sample is irradiated) 2B is clearly included in a first volume of the miniature oven cavity extending in length from somewhere near the rear (inner) face of the plug portion 6A, representing a first end wall, to somewhere near the upper end, or far point, of the slit 2E. The sample vaporizing zone is contained in a second volume extending generally from the first volume (i.e., the target zone) up to the wall portion 2D, representing a second end wall. The effective target zone is included within the hollow cylindrical part 6B and coincides in the main with the volume irradiated by the collimated beam of photons reaching the miniature oven cavity through the passage 6D.

Because of the good thermal conductivity provided from end to end of the working tip 2 by the copper boundary means and the comparatively poor thermal conductivity of the stem tube 3, the whole of the working tip tends to become heated fairly uniformly, with the result that sample condensation on the inside of the boundary means and in the orifice 6D is avoided. To minimize chemical attack, the working tip 2 is suitably protected such as by gold plating both on the inside and the outside.

Although we have found that sufficient heat transfer can be obtained from the heating coil 4A in a layout as described above, means may be provided for supplying additional heat from the free end of the working tip close to the screw plug part 6A.

It has already been observed that sample condensation can be troublesome not only on the critical parts of the working tip 2 passage 6D and slit 2B, in particular but also on the wall of the output passage through which the photons generated by the cooperating plasma discharge lamp must pass. It is clearly desirable, therefore, and in the case of certain samples, almost essential, for the output member in which said passage is provided to be carefully heated so that no part of its bounding wall is likely to experience in operation a temperature lower than the vaporizing temperature of the sample under analysis. This safe-guard has been realized in the FIG. 4 embodiment by causing the plasma-generated heat of a Helium I lamp to heat up the photon output member by thermal conduction therealong, from the inner end exposed to the plasma to the outer end kept in good thermal contact with the boundary means of the working tip 2 (FIG. 1), and preventing the shunting (i.e,, by thermal conduction) of too much heat from the photon output member to the cooling arrangement that forms an essential adjunct of the lamp. If the thermal impedance of the thermal path between the plasma discharge zone of the lamp and the sample vaporizing zone is kept reasonably low and constant by constructing the photon output member and the boundary means of suitably chosen, arranged and proportioned materials, the temperature can be made to fall gradually from the inner end of the output member to the sample vaporizing zone within the working tip of the probe, with the result that if the vaporizing temperature of a sample under analysis is reached at the vaporizing zone the molecules of vaporized sample drifting in the direction of the plasma can never ex perience a temperature lower than the vaporizing temperature.

The above functional outline should aid understanding of the lamp construction to be described with reference to FIG. 4, and the manner in which it is made to cooperate with a sample probe conforming to the FIG. 1 embodiment.

In FIG. 4, a Helium I lamp generally indicated at 8 comprises a photon output member in the form of a thick-walled cylindrical, photon output tube 9 terminating at the lower end in an externally threaded flange 9A and tapering in the upper half of its height to a portion 93 of reduced external diameter. A major cylindrical bore 9C, approximately 6mm in diameter, extends almost the entire length of the photon output tube 9 but at the very end of the portion 98 a transverse wall portion 9D reduces the major bore 9C to a fine orifice 9E, 1mm in diameter. The photon output tube 9 is machined from a solid copper rod to insure good and uniform thermal conductivity.

A boron nitride rod 9F, axially drilled to provide a fine photon output passage 9G, also lmm in diameter, is secured within the major bore 9C, with its upper end abutting against the underside of the transversal wall portion 9D and its photon output passage 9G aligned with the orifice 9E. A radially extending passage 9H penetrates both the wall of the photon output tube 9 and the boron nitride rod 9F to reach the photon output passage 9G.

' The flange 9A of the photon output tube 9 is hollowed out to define a cup-like anode cavity 9I which in operation receives the full blast of the plasma discharge.

The upper portion 9B of the photon output tube 9 is push fit into approximately one half of a locating and retaining sleeve 1A of stainless steel and the working tip 2 of the probe 1, conforming to the FIG. 1 construction, is push fit into the other half. The underside of the plug 6 (see FIG. 1) is made to abut against the top of the transverse wall portion 9D, and the mating surfaces are carefully machined to provide efficient heat transfer therebetween.

The major parts of the Helium I lamp 8 cooperating with the photon output tube 9 are the thermal isolator 10, the water jacket 11, the boron nitride discharge capillary 12 having an outer sheath of aluminum oxide and the cathode chamber 13.

The thermal isolator 10 is made of stainless steel, which is a relatively poor heat conductor, and comprises a thin-walled cylindrical portion 10A extending at the upper end into a comparatively thick annular portion 108 having an internally threaded annular recess 10C into which is screwed the flange 9A of the photon output tube 9. The cylindrical portion 10A is externally threaded at the lower end 10D and screws into an externally threaded upper bore portion 11A of water jacket 11.

The boron nitride discharge capillary 12 is a slide fit in the lower bore portion 113 of the water jacket 11, and passes through the bore of the cylindrical portion 10A with an all-round clearance of approximately 1mm. It terminates closely spaced from the anode cavity 9I, with its capillary passage 12A aligned with the photon output passage 96 and spaced therefrom by a mm gap. A washer 11C having an upstanding inner ridge 11D serves to compress the O-ring 11E against an annular recess HP in the externally threaded boss 116 of water jacket 11 when the ring nut 11H is screwed onto the boss 11G, the O-ring 11E thus forming a vacuum tight seal between the discharge capillary l2 and the lower bore portion 118.

The water jacket 11 comprises, in addition to the parts thereof already described, a mounting flange 111 integral with a body 11J comprising an annular cavity 11K having inlet and outlet connections 11L and 11M, respectively. The flange 11] is radially drilled to form a passage llN corresponding with a drilling 110 in the threaded lower end 10D of the thermal isolator 10. The passage llN extends into stub pipe 11? having connecting end piece llQ, the stub pipe 11? being welded to the flange 111. There is thus established communication with the annular space between the discharge capillary 12 and the bore of the cylindrical portion 10A for differential vacuum pumping purposes, which will be further explained later.

The cathode chamber 13 is a generally cylindrical enclosure of aluminum comprising a Helium inlet 133A at the lower end and a connection 13B at the other (upper) end which allows the discharge capillary 12 to pass through into the cathode chamber 13 while maintaining vacuum tightness in the manner described with reference to the connection (llC-l 1H) positioned immediately above it.

in operation, with the entire lamp and the miniature oven cavity of the sample probe under low subatmospheric pressure, helium gas is leaked into the cathode chamber 13 through the connection 13A. Upon its emergence from the upper end of the capillary passage 12A, the helium gas is for the major part evacuated at a fixed high rate through the annular space between discharge capillary 12 and cylindrical portion 10A, the drilling 110, the passage UN in flange 111, and the stub pipe 1 1?, but some passes through the photon output passage 90 and is evacuated through radial passage 9H. The outer end of passage 9H is of course maintained at vacuum, in particular by the surrounding vacuum chamber enclosing the photo-electron electrostatic analyzer (not shown). By adjusting the rate at which helium is fed into the chamber 13 a plug of slowly moving helium at a suitably low, finely adjusted pressure is held in the plasma discharge zone, which is the space bounded by the wall defining the cavity 91 at one end, the top of the discharge capillary 12 at the other, and the intervening portion of the bore of the cylindrical portion 10A together with its extension into the annular portion 103.

Once the correct helium pressure has been attained in the plasma-discharge zone, a suitably high voltage (the actual starting voltage depends on the purity and pressure of the helium gas and the particular construction of the lamp) is applied between the upper or anode part of the lamp and the lower or cathode part of the lamp, which parts are mechanically linked but electrically insulated through the boron nitride discharge capillary 12. After the plasma discharge has been started, the current through the lamp is adjusted not only to give a good yield of Helium I radiation but also to raise the heat output of the plasma discharge so that enough heat is transmitted through the photon output tube 9 and the boundary means of the working tip 2 to insure that no portion of either the tube 9 or the boundary means experiences a temperature below the vaporizing temperature of the sample under analysis.

With a large number of samples the current through the lamp can be stepped up to the point where the heat finally reaching the sample is sufficient to vaporize it without the assistance of the heated coil 4A in the sample probe 1 (FIG. 1), bearing in mind, however, what has already been said about the possibility and convenience of controlling the final temperature at the sam ple vaporizing zone by means of the heating means in the probe.

On the other hand, the minimum lamp current required to produce a good yield of Helium I radiation may cause too much heat to be transmitted to the sample vaporizing zone, if the vaporization temperature of the particular sample to be analyzed is comparatively low. In extreme cases, the sample might vaporize so quickly that its electron-energy spectrum could not be observed at all. This'contingency is taken care of by controlling the temperature of the thermal isolator 10 by surrounding its cylindrical portion 10A with a thick copper sleeve 14 abutting at one end against the underside of annular portion 108 and held off at the other end from direct contact with an annular step 11R in flange 111 by means of an undulated washer 115, which thus (because of only point or line contact) provides a path of comparatively high thermal impedance to maintain a substantial degree of heat insulation between the sleeve 14 and the water jacket 11, without which the thermal mass of the water jacket 11 would act as a heat sink for the sleeve 14, which could not then be independently temperature controlled. The copper sleeve 14 is radially drilled and tapped at MA to receive a threaded copper stud 148 having a blind bore 14C. A stainless steel tube MD of small bore is folded in U-fashion and located in the bore 14C, with the base of the U near the closed end of the bore. Unions ME and 14F are provided for connecting the tube 14D in a coolant circuit (not shown). A silver solder filling (shown surrounding the U-shaped end of tube 14D) maintains a good thermal contact between the wall of the blind bore 14C and part of the tube 14D which is within it.

A thin-walled stainless steel sleeve 14G fits snugly over the smooth outer cylindrical surface of the stud 1413 at one end and extends at the other end a little beyond the enlarged threaded end 14H of a brass sleeve 14l surrounding stainless steel sleeve 146 with a small cylindrical clearance. An O-ring l4] cooperates with the sleeve 146, an inner groove in end portion 14H and gland nut 14K in maintaining a vacuum seal between sleeves 14G and 14]. The end of the brass sleeve 14] opposite the threaded termination 14H is flared out to define a flange 14L which is forced by gland nut 15A against O-ring 15B seating in a groove 15C in threaded boss 15D of generally parallelopipedal manifold body 15.

A cylindrical bore 15E, the diameter of which is greater than the external diameter of sleeve 14 by 1mm, is provided in body 15. The step 11R fits within the lower end of the bore 15E with a sliding fit. An annular groove is machined in the lower face of body 15 for accommodating therein an O-ring 15F which forms a vacuum seal between said lower face and the upper face of flange 111 when the block 15 is urged against the flange III by the cooperation of four nuts such as 15G threadably engaging the lower ends of four studs such as H depending from the block 15 and passing through holes such as 151 in flange 111. A similar clamping arrangement (not shown) secures the bottom of a vacuum chamber (not shown) within which is accommodated a cylindrical electrostatic analyzer (not shown) to the top of block 15, an intervening O-ring 15] providing a vacuum seal. As a result, the vacuum that in the operation of the photoelectron spectrometer is set up on the vacuum chamber is shared by the interspace between the body 15 and the sleeve 14 and the interspace between the brass sleeve 141 and the thin stainless steel sleeve 14G, communication between the two interspaces being established by virtue of the fact that the threaded boss ISD is bored right through to a diameter exceeding by 1mm the outside diameter of the stainless steel sleeve 146. The purpose of the vacuum interspace is to prevent, as much as possible, heat transfer between the sleeve 14 and its surroundings other than through the action of the coolant passing through tube 14D.

The cooling arrangement described above clearly serves to proportion the amount of plasma-generated heat routed through the photon output tube 9. By circulating through the tube 14D a heating rather than a cooling fluid, it is naturally possible to boost the transmitted heat, if this is in fact desirable in dealing with analytical samples vaporizing at exceptionally high temperatures. Broadly speaking, therefore, the arrangement may be regarded as a heat exchanger for either reducing (usually) or supplementing the plasmagenerated heat transmitted through the photon output tube 9.

It has been observed earlier that photoelectron spectrometry is now an important tool in fundamental research. The research analyst obviously deals with many samples of unknown characteristics, in particular samples which may well vaporize at a temperature not very much higher than ambient temperature. In the operation of the apparatus described with reference to FIG. 4, the user may find it desirable for such samples to run the Helium I lamp so as to achieve in the first instance the lowest heat transfer through the tube 9 to the working tip 2 that can be obtained by (a) keeping the energization of the lamp as low as is consistent with an acceptable yield of Helium I radiation, and (b) passing an efficient coolant through the tube 14D at the highest permissible rate. He would then gradually allow the sample to reach its vaporizing temperature by carefully controlling the heater current in the heater 4 (FIG. 1).

It is possible in unusual situations that, after taking all the practical steps to minimize heat transfer from the plasma-discharge zone to the sample'zone, the heat reaching the sample is still sufficient to cause rapid vaporization of the (low boiling point) sample. In these circumstances, control over the sample temperature may be regained by withdrawing the inner tube 5 carrying the heater 4 (FIG. 1) from the probe 1 and substituting a similar tube 16 (FIG. 5) fitted with a copper end shield 16A, closed at the bottom, within which is accommodated a cooling tube 168 bent in the shape of a U, the root of the U making good contact with the bottom of the shield 16A through a silver solder filling 16D. Unions 16B and 16F enable the tube 16B to be inserted in a cooling circuit. By controlling the rate of coolant flow the sample temperature can be adjusted to obtain the most efficient utilization of the sample available.

The use of the substitute stern tube 16 depicted in FIG. 5 may be extended with advantage to a situation where the sample vaporizes at a temperature which is only slightly below ambient temperature so that it is difficult to exercise fine control on its rate of vaporization. The cooling arrangement could be used to keep the sample well below vaporization temperature while the sample is accommodated in the sample holder 7 (FIG. 2) and the analyst is about to scan its photoelectron spectrum. At the moment a spectrum is to be run, the analyst would gradually allow the sample to heat up until a spectrum began to appear, e.g., on a chart recorder.

The circuit diagram of FIG. 6 shows the simple electrical energization and control arrangement for the embodiment of FIG. 4. The current through the Helium I lamp 8 is regulated by means of a rheostat 17 in series with a DC supply 18 and the current through the heater coil 4A is regulated by a rheostat 19 in series with an AC supply 20. At least rheostat 19 may be servolooped so as to be controlled by the thermal measuring element (e.g., 4C in FIG. 1).

Although the foregoing description of the FIG. 4 embodiment indicates a mode of operation in which the parts mounted at the top end of the capillary discharge tube 12 are electrically positive relative to the chamber 13, it is possible to operate the Helium I lamp with reverse polarity.

It will be observed that whether the Helium I lamp 8 (FIG. 4) is allowed to produce maximum or minimum heat transfer to the sample zone, no sample molecules drifting in the direction of the plasma-discharge zone can encounter temperatures that are below the vaporizing temperature of the sample. With the arrangements described the moment molecules of sample enter the gas phase they are bound to become hotter and hotter as they drift downwards towards the plasma, which means that they do not condense along their travel so that they are finally decomposed by reaching the plasma.

What we claim is:

1. Apparatus for vaporizing liquid or solid samples in photoelectron spectrometry, comprising:

heat conducting boundary means for defining therein a miniature sample oven cavity providing a sample vaporizing zone contiguous to a target zone;

a photon entry aperture and a photoelectron exit aperture in said boundary means, into and out of said target zone, respectively;

and heating means in close thermal transfer relationship with said boundary means,

whereby the all-round heat emanating from the surrounding boundary means into the confined space of the miniature oven cavity assists the formation of a uniform molecular cloud'of sample diffusing with random agitation throughout the oven cavity and minimizes the risk of sample condensation.

2. Apparatus as claimed in claim 1, wherein:

said boundary means comprises a longitudinally extending tubular wall, a first end wall and a second end wall.

3. Apparatus as claimed in claim 2, wherein:

said photon entry aperture is provided in the first end wall;

said photoelectron exit aperture is a slit in said longitudinally extending tubular wall parallel to the axis thereof starting at a point near said first end wall and terminating at a predetermined far point;

said target zone is included in a first volume of the miniature oven cavity between said first end wall and substantially said far point;

and said sample vaporizing zone is included in a second volume forming a longitudinal extension of longitudinal slot registering with said photon exit aperture.

7. Apparatus as claimed in claim 5, wherein:

said extension is adapted to support a sample holder and cooperates with said sample holder and said second end wall to define a restricted annular passage between the sample holder and the second end wall through which molecules of vaporized sample diffuse into the miniature oven cavity.

8. Apparatus as claimed in claim 3, wherein:

said boundary means is part of a sample probe working tip.

9. Apparatus as claimed in claim 8, wherein:

said heating means is part of the sample probe and is located near said vaporizing zone.

10. Apparatus as claimed in claim 8, wherein:

said heating means is adapted for heat transfer contact with said working tip of the sample probe, but is separable from the other parts of said working tip.

11. Apparatus for vaporizing liquid or solid samples in a photoelectron spectrometer comprising:

heat conducting boundary means for defining therein a miniature sample oven cavity providing a sample vaporizing zone contiguous to a target zone;

a photon entry aperture and a photoelectron exit aperture in said boundary means, into and out of said target zone, respectively;

means for supporting a sample within said sample vaporizing zone;

and a plasma-discharge photon source having a photon heat conducting output member exposed at one end to the blast of the plasma discharge and making good thermal contact at the other end with said boundary means;

said photon output member being provided with a photon output passage which is aligned with said photon entry aperture and being so constituted and arranged as to enable a substantial part of the plasma-generated heat to'be transmitted through said photon output member to said boundary means,

whereby in operation plasma-generated heat is transmitted through the photon output member and the boundary means to the sample vaporizing zone and the temperature encountered by any sample molecules drifting from the sample vaporizing zone towards the plasma discharge is prevented from falling below the vaporizing temperature of the sample.

12. Apparatus as claimed in claim 11, further comprising:

means for regulating the electrical energization of said plasma-discharge source and thus the heat output of the plasma.

13. Apparatus as claimed in claim 12, further comprising:

controllable heating means provided to close to said sample vaporizing zone for the purpose of complementing the plasma-generated heat reaching the sample.

14. Apparatus as claimed in claim 12, further comprising:

cooling means provided close to said sample vaporizing zone for the purpose of detracting from the plasma generated heat reaching the sample.

15. Apparatus as claimed in claim 11, wherein said plasma-discharge photon source is a capillary plasma-discharge source in which an electrically insulating and heat resisting capillary tube communicates with an electrode chamber at each end.

16. Apparatus asclaimed in claim 15 wherein:

said photon output member partly defines the anode chamber of the plasma-discharge photon source.

17. Apparatus as claimed in claim 15, wherein:

said capillary tube is surrounded by a cooling device.

18. Apparatus as claimed in claim 11, wherein:

said heat conducting boundary means form part of a sample probe.

19. Apparatus as claimed in claim 11, further comprising:

a heat exchanger means provided around that part of the photon source wherein the plasma discharge takes place for modifying the amount of the plasma-generated heat transmitted through said photon output member.

20. Apparatus as claimed in claim 11, wherein:

said photon output member is supported on thermal isolating means.

21. Apparatus as claimed in claim 20, further comprising:

heat exchanger means adjacent to and operationally associated with said thermal isolating means for modifying the amount of the plasma-generated heat transmitted through the photon output mem- 

1. Apparatus for vaporizing liquid or solid samples in photoelectron spectrometry, comprising: heat conducting boundary means for defining therein a miniature sample oven cavity providing a sample vaporizing zone contiguous to a target zone; a photon entry aperture and a photoelectron exit aperture in said boundary means, into and out of said target zone, respectively; and heating means in close thermal transfer relationship with said boundary means, whereby the all-round heat emanating from the surrounding boundary means into the confined space of the miniature oven cavity assists the formation of a uniform molecular cloud of sample diffusing with random agitation throughout the oven cavity and minimizes the risk of sample condensation.
 2. Apparatus as claimed in claim 1, wherein: said boundary means comprises a longitudinally extending tubular wall, a first end wall and a second end wall.
 3. Apparatus as claimed in claim 2, wherein: said photon entry aperture is provided in the first end wall; said photoelectron exit aperture is a slit in said longitudinally extending tubular wall parallel to the axis thereof starting at a point near said first end wall and terminating at a predetermined far point; said target zone is included in a first volume of the miniature oven cavity between said first end wall and substantially said far point; and said sample vaporizing zone is included in a second volume forming a longitudinal extension of said first volume up to said second end wall.
 4. Apparatus as claimed in claim 2, wherein: said first end wall is removable.
 5. Apparatus as claimed in claim 4, wherein: said first end wall is provided with an extension for supporting a sample in said sample vaporization zone.
 6. Apparatus as claimed in claim 5, wheRein: said extension is a hollow cylinder provided with a longitudinal slot registering with said photon exit aperture.
 7. Apparatus as claimed in claim 5, wherein: said extension is adapted to support a sample holder and cooperates with said sample holder and said second end wall to define a restricted annular passage between the sample holder and the second end wall through which molecules of vaporized sample diffuse into the miniature oven cavity.
 8. Apparatus as claimed in claim 3, wherein: said boundary means is part of a sample probe working tip.
 9. Apparatus as claimed in claim 8, wherein: said heating means is part of the sample probe and is located near said vaporizing zone.
 10. Apparatus as claimed in claim 8, wherein: said heating means is adapted for heat transfer contact with said working tip of the sample probe, but is separable from the other parts of said working tip.
 11. Apparatus for vaporizing liquid or solid samples in a photoelectron spectrometer comprising: heat conducting boundary means for defining therein a miniature sample oven cavity providing a sample vaporizing zone contiguous to a target zone; a photon entry aperture and a photoelectron exit aperture in said boundary means, into and out of said target zone, respectively; means for supporting a sample within said sample vaporizing zone; and a plasma-discharge photon source having a photon heat conducting output member exposed at one end to the blast of the plasma discharge and making good thermal contact at the other end with said boundary means; said photon output member being provided with a photon output passage which is aligned with said photon entry aperture and being so constituted and arranged as to enable a substantial part of the plasma-generated heat to be transmitted through said photon output member to said boundary means, whereby in operation plasma-generated heat is transmitted through the photon output member and the boundary means to the sample vaporizing zone and the temperature encountered by any sample molecules drifting from the sample vaporizing zone towards the plasma discharge is prevented from falling below the vaporizing temperature of the sample.
 12. Apparatus as claimed in claim 11, further comprising: means for regulating the electrical energization of said plasma-discharge source and thus the heat output of the plasma.
 13. Apparatus as claimed in claim 12, further comprising: controllable heating means provided close to said sample vaporizing zone for the purpose of complementing the plasma-generated heat reaching the sample.
 14. Apparatus as claimed in claim 12, further comprising: cooling means provided close to said sample vaporizing zone for the purpose of detracting from the plasma generated heat reaching the sample.
 15. Apparatus as claimed in claim 11, wherein: said plasma-discharge photon source is a capillary plasma-discharge source in which an electrically insulating and heat resisting capillary tube communicates with an electrode chamber at each end.
 16. Apparatus as claimed in claim 15 wherein: said photon output member partly defines the anode chamber of the plasma-discharge photon source.
 17. Apparatus as claimed in claim 15, wherein: said capillary tube is surrounded by a cooling device.
 18. Apparatus as claimed in claim 11, wherein: said heat conducting boundary means form part of a sample probe.
 19. Apparatus as claimed in claim 11, further comprising: a heat exchanger means provided around that part of the photon source wherein the plasma discharge takes place for modifying the amount of the plasma-generated heat transmitted through said photon output member.
 20. Apparatus as claimed in claim 11, wherein: said photon output member is supported on thermal isolating means.
 21. Apparatus as claimed in claim 20, further comprising: heat exchanger means adjacent to and operationally associated with said therMal isolating means for modifying the amount of the plasma-generated heat transmitted through the photon output member. 